简单英语学Rust

接受用户输入

一个简单的方法是用std::io::stdin来接受用户的输入。这意味着 "标准输入"，也就是来自键盘的输入。用stdin()可以获得用户的输入，但是接下来你就会想用.read_line()把它放到&mut String中。下面是一个简单的例子，但它既能工作，也不能工作:

use std::io;

fn main() {
    println!("Please type something, or x to escape:");
    let mut input_string = String::new();

    while input_string != "x" { // This is the part that doesn't work right
        input_string.clear(); // First clear the String. Otherwise it will keep adding to it
        io::stdin().read_line(&mut input_string).unwrap(); // Get the stdin from the user, and put it in read_string
        println!("You wrote {}", input_string);
    }
    println!("See you later!");
}

下面是一个输出输出的样子。

Please type something, or x to escape:
something
You wrote something

Something else
You wrote Something else

x
You wrote x

x
You wrote x

x
You wrote x

它接受我们的输入，然后把它还给我们，它甚至知道我们输入了x。但它并没有退出程序。唯一的办法是关闭窗口，或者输入ctrl和c。让我们把println!中的{}改为{:?}，以获得更多的信息(如果你喜欢那个宏，也可以使用dbg!(&input_string))。现在它说

Please type something, or x to escape:
something
You wrote "something\r\n"
Something else
You wrote "Something else\r\n"
x
You wrote "x\r\n"
x
You wrote "x\r\n"

这是因为键盘输入其实不只是something，而是something和Enter键。有一个简单的方法可以解决这个问题，叫做.trim()，它可以把所有的空白都去掉。顺便说一下，这些字符都是空白字符。

U+0009 (horizontal tab, '\t')
U+000A (line feed, '\n')
U+000B (vertical tab)
U+000C (form feed)
U+000D (carriage return, '\r')
U+0020 (space, ' ')
U+0085 (next line)
U+200E (left-to-right mark)
U+200F (right-to-left mark)
U+2028 (line separator)
U+2029 (paragraph separator)

这样就可以把x\r\n变成只剩x了。现在它可以工作了:

use std::io;

fn main() {
    println!("Please type something, or x to escape:");
    let mut input_string = String::new();

    while input_string.trim() != "x" {
        input_string.clear();
        io::stdin().read_line(&mut input_string).unwrap();
        println!("You wrote {}", input_string);
    }
    println!("See you later!");
}

现在可以打印了:

Please type something, or x to escape:
something
You wrote something

Something
You wrote Something

x
You wrote x

See you later!

还有一种用户输入叫std::env::Args(env是环境的意思)。Args是用户启动程序时输入的内容。其实在一个程序中总是至少有一个Arg。我们写一个程序，只用std::env::args()来打印它们，看看它们是什么。

fn main() {
    println!("{:?}", std::env::args());
}

如果我们写cargo run，那么它的打印结果是这样的:

Args { inner: ["target\\debug\\rust_book.exe"] }

让我们给它更多的输入，看看它的作用。我们输入 cargo run but with some extra words 。 它给我们:

Args { inner: ["target\\debug\\rust_book.exe", "but", "with", "some", "extra", "words"] }

有意思。而当我们查看Args的页面时，我们看到它实现了IntoIterator。这意味着我们可以.用所有我们知道的关于迭代器的方法来读取和改变它。让我们试试这个:

use std::env::args;

fn main() {
    let input = args();

    for entry in input {
        println!("You entered: {}", entry);
    }
}

现在它说:

You entered: target\debug\rust_book.exe
You entered: but
You entered: with
You entered: some
You entered: extra
You entered: words

你可以看到，第一个参数总是程序名，所以你经常会想跳过它，比如这样:

use std::env::args;

fn main() {
    let input = args();

    input.skip(1).for_each(|item| {
        println!("You wrote {}, which in capital letters is {}", item, item.to_uppercase());
    })
}

这将打印:

You wrote but, which in capital letters is BUT
You wrote with, which in capital letters is WITH
You wrote some, which in capital letters is SOME
You wrote extra, which in capital letters is EXTRA
You wrote words, which in capital letters is WORDS

Args的一个常见用途是用于用户设置。你可以确保用户写出你需要的输入，只有在正确的情况下才运行程序。这里有一个小程序，可以让字母变大(大写)或变小(小写)。

use std::env::args;

enum Letters {
    Capitalize,
    Lowercase,
    Nothing,
}

fn main() {
    let mut changes = Letters::Nothing;
    let input = args().collect::<Vec<_>>();

    if input.len() > 2 {
        match input[1].as_str() {
            "capital" => changes = Letters::Capitalize,
            "lowercase" => changes = Letters::Lowercase,
            _ => {}
        }
    }

    for word in input.iter().skip(2) {
      match changes {
        Letters::Capitalize => println!("{}", word.to_uppercase()),
        Letters::Lowercase => println!("{}", word.to_lowercase()),
        _ => println!("{}", word)
      }
    }

}

下面是它给出的一些例子。

输入: cargo run please make capitals:

make capitals

输入:cargo run capital:

// Nothing here...

输入:cargo run capital I think I understand now:

I
THINK
I
UNDERSTAND
NOW

输入:cargo run lowercase Does this work too?

does
this
work
too?

除了用户给出的 Args，在 std::env::args() 中可用，还有系统变量Vars。这些都是用户没有输入的程序的基本设置。你可以用std::env::vars()把它们都看成一个(String, String)。这个有非常多，比如说:

fn main() {
    for item in std::env::vars() {
        println!("{:?}", item);
    }
}

运行这段代码，就能显示出你的用户会话的所有信息。它将显示这样的信息:

("CARGO", "/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/cargo")
("CARGO_HOME", "/playground/.cargo")
("CARGO_MANIFEST_DIR", "/playground")
("CARGO_PKG_AUTHORS", "The Rust Playground")
("CARGO_PKG_DESCRIPTION", "")
("CARGO_PKG_HOMEPAGE", "")
("CARGO_PKG_NAME", "playground")
("CARGO_PKG_REPOSITORY", "")
("CARGO_PKG_VERSION", "0.0.1")
("CARGO_PKG_VERSION_MAJOR", "0")
("CARGO_PKG_VERSION_MINOR", "0")
("CARGO_PKG_VERSION_PATCH", "1")
("CARGO_PKG_VERSION_PRE", "")
("DEBIAN_FRONTEND", "noninteractive")
("HOME", "/playground")
("HOSTNAME", "f94c15b8134b")
("LD_LIBRARY_PATH", "/playground/target/debug/build/backtrace-sys-3ec4c973f371c302/out:/playground/target/debug/build/libsqlite3-sys-fbddfbb9b241dacb/out:/playground/target/debug/build/ring-cadba5e583648abb/out:/playground/target/debug/deps:/playground/target/debug:/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib:/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib")
("PATH", "/playground/.cargo/bin:/playground/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin")
("PLAYGROUND_EDITION", "2018")
("PLAYGROUND_TIMEOUT", "10")
("PWD", "/playground")
("RUSTUP_HOME", "/playground/.rustup")
("RUSTUP_TOOLCHAIN", "stable-x86_64-unknown-linux-gnu")
("RUST_RECURSION_COUNT", "1")
("SHLVL", "1")
("SSL_CERT_DIR", "/usr/lib/ssl/certs")
("SSL_CERT_FILE", "/usr/lib/ssl/certs/ca-certificates.crt")
("USER", "playground")
("_", "/usr/bin/timeout")

所以如果你需要这些信息，Vars就是你想要的。

获得单个Var'的最简单方法是使用env!宏。你只要给它变量的名字，它就会给你一个&str'的值。如果变量拼写错误或不存在，它就不起作用，所以如果你不确定，就用option_env!代替。如果我们在Playground上写这个:

fn main() {
    println!("{}", env!("USER"));
    println!("{}", option_env!("ROOT").unwrap_or("Can't find ROOT"));
    println!("{}", option_env!("CARGO").unwrap_or("Can't find CARGO"));
}

然后我们得到输出:

playground
Can't find ROOT
/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/cargo

所以option_env!永远是比较安全的宏。如果你真的想让程序在找不到环境变量时崩溃，那么env!会更好。